In the 2010 general election, 35% of registered voters did not cast a ballot. Who are the "unheard third" who did not vote - and what could their role be in the coming election?
In central Manchester, there are few clues that the election is just days away.
Flags pledging allegiance to either Manchester City or Manchester United football teams hang in people's windows.
Posters pledging a political allegiance are harder to spot.
The turnout in this constituency, Manchester Central, was the lowest of anywhere in the UK in the last election - only 44% of those who registered actually voted.
The last MP for the area, Labour's Lucy Powell, was elected after a by-election in 2012 on a turnout of just 18%, thought to be the lowest ever.
She has admitted feeling "embarrassed and horrified" by the record.
But why is turnout here so low?
'Web of lies'
In a small street market in Moss Side, in the south of the constituency, it seems anger not apathy is turning people off voting.
Carer Shirley says she used to vote for Labour, but hasn't voted for at least 18 years.
She insists no-one will change her mind now.
"In inner-city areas people are suffering more [now] than 10 years ago. Why should we vote for this?" she says.
"They spin a web of lies, and then six months later they're doing the complete opposite. Just let them get on with it.
"When campaigners knock my door, I don't answer. Not interested. We don't vote."
Tony Bellows, a 55-year-old civil servant who lives nearby in Hulme, agrees.
"They're just a bunch of liars. They're just puppets putting a story forward to grab your eye," he says.
"That's what I teach my kids. They've totally broken all trust."
The issue of broken trust is a common refrain here; as are feelings that "all politicians are the same" and that "voting won't make any difference" - either because their vote is one amongst millions, or because of a belief that the system is corrupt.
And it's a trend that seems to have played out among non-voters across the country.
A Survation poll which questioned people who didn't vote in the 2010 election found the sentiments cited by those on the streets of Manchester were among the top three reasons people didn't go to the ballot box.
Only 6% of non-voters sampled trusted politicians to tell the truth - compared with 9% of voters.
"A deeply disillusioned citizenry that will be hard to motivate" is how the Hansard Society described the British electorate in its audit of political engagement earlier this year.
It predicts that, as in 2010, turnout will be lowest among ethnic minorities, people who rent rather than own their home, and those on lower incomes.
The UK's lowest ever election turnout was 57% in 1918, just after World War One. It reached a peak of 83% in 1950, and has seemingly been on a slow but steady decline since.
But are warnings about the gradual death of democracy and rise of political apathy from some quarters to be heeded?
Professor of British Politics at Cardiff University Pete Dorey argues not.
He predicts turnout may be higher on 7 May because it's been such a close-run race.
"More people feel their vote might count more," he says.
"When one party is miles ahead, even their own supporters don't necessarily turn out.
"I think turnout in Scotland will be high because of the referendum. I think people will feel energised to vote."
He says some people may be suffering from "democracy fatigue" because of the frequency of elections - council, EU, mayoral, police commissioner - but he says it's a "dangerous assumption" to think all non-voters are apathetic.
"It can be apathy, but can be a perfectly rational decision. And yes, that's just as much of a democratic choice as not voting.
Can't vote, won't vote
- Under 18s
- Members of the House of Lords
- Prisoners (apart from remand prisoners)
- Jehovah's Witnesses
- The Royal family
Find out why: The thousands who will not vote
BBC News Timeliner
Winners and losers. Laughter and tears. Celebrities and the general public. Find echoes of the current campaign on a trip through the archive of elections past.
"I don't agree that we should be like Australia and make voting compulsory."
But he says even though he can understand the "Russell Brand's previous anti-voting stance", he thinks people still ought to vote.
"Even if that party doesn't win, they get a higher proportional share of the vote, and that sends a signal. Politicians will simply ignore those who don't vote."
In such a tightly-fought election, getting voters mobilised on election day will be crucial for all parties - even in seats where turnout is traditionally high.
East Renfrewshire had the highest voter turnout in 2010 at 77%.
Here Scottish Labour leader Jim Murphy is fighting to keep his seat; a poll by Lord Ashcroft released on Friday has him trailing three points behind the SNP.
Of all the parties, Professor Dorey says turnout is likely to play the biggest role for Labour.
Three-quarters of those who say they support the Conservative Party or UKIP also say they are certain to vote, compared to 52% of those who support the Labour Party, the Hansard Society found.
But what can parties do to mobilise that untapped seam of voters?
Fran O'Leary, from communications company Lodestone, which commissioned the Survation survey of non-voter attitudes, thinks parties should look to the new style of "neighbour-to-neighbour" campaigning that's emerged in the US.
She worked on the Obama campaign in Las Vegas in 2012.
"I came back to the UK with a really strong sense that something really exciting had happened there," she says.
"It's a more conversational politics - rather than the traditional way of campaigning, which is to tell them your five key messages."
Perhaps the biggest challenge for all parties, however, is getting people aged between 18 and 24 out to vote.
Only 39% of women and 50% of men in that age group cast a ballot in the last election, Ipsos Mori research found.
Campaigns such as Bite the Ballot have had success using social media to try to persuade more young people to register to vote - but will they turn out on the day?
Back in Manchester, friends Megan Burns and Ceire Lennon, both 21, are taking a break outside the university library.
This is the first election they are old enough to vote in, and both have registered - but both are unsure whether they'll use their vote.
"No-one represents us," says bio-chemistry student Megan. "I thought about abstaining, but I feel like it's a waste. I'm a feminist and I feel like my mum and grandmother would be disappointed."
This will also be 18-year-old A level student Louis Registe's first election as an eligible voter.
"I wanted to register to vote to express a preference, but I don't know if I will vote. The more I try to find out in order to decide, the more confused I become.
"I've watched all the debates... but I feel like I don't know what I'm voting for with any of them."